1. Field of the Invention
The present invention relates to a signal detector to detect a single tone of a single frequency sinusoidal wave inclusive of a phase change.
2. Description of the Related Art
FIG. 1 shows a block diagram of a narrow-band signal detector disclosed in Japanese Patent Application Laid-Open No. 295422/91.
Digital signal x(t) is input to unit delay circuit 401 and subtracter 403 from input terminal 413. Unit delay circuit 401 delays input digital signal x(t) by one sample time, and outputs the delayed input digital signal to unit delay circuit 402 and multiplier 404. Multiplier 404 multiplies the output of unit delay circuit 401 by output a.sub.1 of filter coefficient modification unit 407. Meanwhile, the output of unit delay circuit 402 is input to multiplier 405, and is multiplied by output a.sub.2 of filter coefficient modification unit 408.
Next, the outputs of multipliers 404 and 405 are summed up by adder 406 and the resultant signal is input to subtracter 403 as an estimation signal. Subtracter 403 subtracts the estimation signal from input digital signal x(t), with the output being accepted as a residual difference signal. The outputs of both filter coefficient modification units 407 and 408 are modified adaptively based on the residual difference signal.
Signal-to-noise (hereinafter referred to as S/N) ratio calculator 409 calculates a signal power ratio between the audio part and audio-free part of input digital signal x(t). Then, threshold selector 410 selects a threshold corresponding to the output of S/N ratio calculator 409 from a plurality thresholds of, and outputs it. Judgment unit 411 compares filter adaptive coefficient a.sub.2, which is an output of filter coefficient modification unit 408, with a variable threshold approximate to -1 corresponding to the S/N ratio, which is the output of threshold selector 410, to judge whether or not the input digital signal is a single tone. When filter adaptive coefficient a.sub.2 is below the threshold, judgment unit 41 determines that the input is a single tone, and outputs a single tone detection signal from output terminal 412.
In the narrow-band signal detector shown in FIG. 1, an input signal is estimated, based on past input signals, using an adaptive filter. If filter coefficient a.sub.2 have converged to 1, input signal x(t) is determined as a narrow-band signal. Further, when the calculated S/N of the input signal is low, the threshold to be applied for judgment is changed. Thus detection of a narrow-band signal is possible even when the S/N ratio is low.
FIG. 2 shows a block diagram of a detector to detect a narrow-band signal of a desired frequency inclusive a phase change. The detector has a required number of band pass filters corresponding to desired frequencies and judgment units.
Input signal x(t) from input terminal 531, is input to each of the band pass filters 501.about.50n. Judgment units 511.about.51n compare each input signal of each of the band pass filters 501.about.50n with the output signal therefrom. If the result of the comparison shows a small difference, a judgment is made that there has been an input of the in-band frequency signal of the band pass filter concerned.
In detecting a phase change, the point where a level-compared value changes is noted. Since the instant some phase deviation takes place, input signal x(t) becomes an out-band frequency signal, a level difference occurs between each input and output of band pass filters 501.about.50n. Phase deviation data is detected at this instant. Detection circuit 520 detects from the outputs of respective judgment units 511.about.51n that there has been an input of the signal of a desired frequency and subsequently outputs the result to output terminal 532.
The large concerns in realizing a signal detector involve the amount of arithmetic operation and the accuracy of detecting low S/N signals.
Example 1 of the prior art (FIG. 1) shows the following problems:
Firstly, as the prior art aims to detect a narrow-band signal, it can not specify the frequency of a narrow-band signal.
Secondly, due to lack of a function to control the amount of modification serviceable for filter coefficient alteration, the filter coefficient modification amount converges unstably with low S/N signals. This makes it necessary to monitor the S/N ratio and then change a judgment threshold when the S/N ratio is found to be low. As a result, accuracy of detection is lowered.
Example 2 of the prior art (FIG. 2) shows the following problems:
Firstly, in a case where a plurality of frequencies are the objects of detection, a plurality of band pass filters must be provided. Each filter must inputs processes every sampled and data, thus resulting in the increase of the amount of overall arithmetic operation within a unit time.
Secondly, a judgment by means of a level comparison between the input to and the output from a band pass filter, causes an error when the input signal is of a low S/N. As a result, detection becomes difficult with low S/N input signals.
Thirdly, as regards detection of phase change, though a phase change point is detectable, the amount of a change in phase is not detectable.